Solid state emitters such as LEDs or lasers may be used to provide white light (e.g., perceived as being white or near-white), and have been investigated as potential replacements for white incandescent lamps. Solid state emitters may include lumiphoric materials (also known as lumiphors) that absorb a portion of emissions having a first peak wavelength emitted by the emitter and re-emit light having a second peak wavelength that differs from the first peak wavelength. Phosphors, scintillators, and lumiphoric inks are common lumiphoric materials. Light perceived as white or near-white may be generated by a combination of red, green, and blue (“RGB”) emitters, or, alternatively, by combined emissions of a blue light emitting diode (“LED”) and a lumiphor such as a yellow phosphor (e.g., YAG:Ce or Ce:YAG). In the latter case, a portion of the blue LED emissions pass through the phosphor, while another portion of the blue emissions is downconverted to yellow, and the blue and yellow light in combination are perceived as white. White light may also be produced by stimulating phosphors or dyes of multiple colors with a violet or UV LED source.
It is known to enclose an LED chip in a package to provide environmental and/or mechanical protection, color selection, light focusing and other functions. A LED package also includes electrical leads, contacts, and/or traces for electrically connecting the LED package to an external circuit. A conventional LED package 20 is illustrated in FIG. 1, including one or more LED chips 22 mounted over a carrier such as a printed circuit board (PCB) carrier, substrate or submount 23, which may include ceramic material. The package 20 may include one or more LED chips 22 of any suitable spectral output (e.g., ultraviolet, blue, green, red, white (e.g., blue LED chip arranged to stimulate emissions of phosphor material) and/or other colors). A reflector 24 may be mounted on the submount 23 (e.g., with solder or epoxy) to surround the LED chip(s) 22, reflect light emitted by the LED chips 22 away from the package 20, and also provide mechanical protection to the LED chips 22. One or more wirebond connections 21 may be made between ohmic contacts on the LED chips 22 and electrical traces 25A, 25B on the submount 23. The LED chips 22 are covered with a transparent encapsulant 26, which may provide environmental and mechanical protection to the chips while also acting as a lens.
Emissions of a blue LED in combination with a yellow or green lumiphoric material may be near-white in character and referred to as “blue-shifted yellow” (“BSY”) light or “blue-shifted green” (“BSG”) light. Addition of red spectral output from a red-emitting LED (to yield a “BSY+R” device) or from a red lumiphoric material (to yield a “BS(Y+R)” device) may be used to increase the warmth of the aggregated light output and better approximate light produced by incandescent lamps.
Quality artificial lighting generally attempts to emulate the characteristics of natural light. Natural light sources include daylight with a relatively high color temperature (e.g., ˜5000K) and incandescent lamps with a lower color temperature (e.g., ˜2800K).
Color reproduction is commonly measured using Color Rendering Index (CRI) or average Color Rendering Index (CRI Ra). To calculate CRI, the color appearance of 14 reflective samples is simulated when illuminated by a reference radiator (illuminant) and the test source. The general or average color rendering index CRI Ra is a modified average utilizing the first eight indices, all of which have low to moderate chromatic saturation. (R9 is one of six saturated test colors not used in calculating CRI, with R9 embodying a large red content.) CRI and CRI Ra are used to determine how closely an artificial light source matches the color rendering of a natural light source at the same correlated color temperature. Daylight has a high CRI Ra (approximately 100), with incandescent bulbs also being relatively close (CRI Ra greater than 95), and fluorescent lighting being less accurate (with typical CRI Ra values of approximately 70-80).
CRI Ra (or CRI) alone is not a satisfactory measure of the benefit of a light source, since it confers little ability to predict color discrimination (i.e., to perceive subtle difference in hue) or color preference. There appears to be a natural human attraction to brighter color. Daylight provides a spectrum of light that allows the human eye to perceive bright and vivid colors, which allows objects to be distinguished even with subtle color shade differences. Accordingly, it is generally recognized that daylight is the “best” light for emphasizing and distinguishing color. The ability of human vision to differentiate color is different under correlated color temperature conditions providing the same CRI Ra. Such differentiation is proportional to the gamut of the illuminating light.
Gamut area of a light source can be calculated as the area enclosed within a polygon defined by the chromaticities in CIE 1976 u′v′ color space of the eight color chips used to calculate CRI Ra when illuminated by a test light source. Gamut area index (GAI) is a convenient way of characterizing in chromaticity space how saturated the illumination makes objects appear—with a larger GAI making object colors appear more saturated. GAI is a relative number whereby an imaginary equal-energy spectrum (wherein radiant power is equal at all wavelengths) is scored as 100. GAI for a test source is determined by comparing color space area of the light being tested to the color space area produced by the imaginary or theoretical equal-energy spectrum (EES) source. Unlike CRI Ra (or CRI), which has a maximum value of 100, GAI can exceed 100, meaning that some sources saturate colors more than an equal-energy source serves to saturate color.
It is found that typical blackbody-like light sources and typical daylight-like light sources have different gamut areas. Low correlated color temperature (CCT) sources (e.g., incandescent emitters) have a gamut area index of approximately 50% (i.e., about half the gamut area of the EES source). Sources with higher CCT values have a larger GAI. For example, a very bluish light with a CCT of 10000K may have a GAI of 140%.
The reference spectra used in color rendering index calculations were chosen as ideal illumination sources defined in terms of their color temperature. As a heated object becomes incandescent, it first glows reddish, then yellowish, then white, and finally bluish. Thus, apparent colors of incandescing materials are directly related to their actual temperature (in Kelvin (K). Practical materials that incandesce are said to have correlated color temperature (CCT) values that are directly related to color temperatures of blackbody sources. CCT is intended to characterize the apparent “tint” of the illumination (e.g., warm or cool) produced by an electric light source. Certain implicit assumptions are embedded in this CCT designation—such as the assumption that chromaticities along the line of blackbody radiation are perceived as ‘white’, and that a CCT designation for a manufactured light source implies consistency in chromaticities of all sources having that designation. Recent research suggests, however, that most sources with chromaticities along the line of blackbody radiation do not appear “white”; rather, such sources provide illumination with discernible tint. An empirically established line of minimum tint in CIE 1931 (x,y) chromaticity space for CCTs between 2700K and 6500K is shown in FIG. 2 Researchers have determined that a majority of people prefer sources of illumination on this “white body line” (i.e., line of minimum tint) more than those of the same CCT line of blackbody radiation. (See, e.g., Rea, M. S. and Freyssinier, J. P.: White lighting for residential applications, Light Res. Tech., 45(3), pp. 331-344 (2013).) As shown in FIG. 2, at CCT values below about 4000K, the “white body line” (WBL) is below the blackbody curve, whereas at higher CCT values, the WBL is above the blackbody curve.
Rea and Freyssinier have proposed that lighting could be generally improved by ensuring that its CRI Ra value is at least 80 while its GAI is in a range of from 80 to 100 (i.e., from 80% to 100% of an equal energy spectrum).
Characteristics including CCT, CRI Ra, GAI, CIE 1931 (x,y) coordinates, luminous efficacy (lm/W), and scotopic/photopic (S/P) ratios for eighteen different light sources are tabulated in FIG. 3A, and chromaticities for selected sources of the foregoing eighteen light sources are plotted in FIG. 3B together with the blackbody curve and the WBL (line of minimum tint). (Source: “Value Metrics for Better Lighting,” Rea, Mark S., et al., 2013, pp. 54 & 63, SPIE Press (Bellingham, Wash., US), ISBN 978-0-8194-9322-4.) As indicated in FIG. 3A, the ability of artificial lights to accurately illuminate color objects varies enormously by type. Solid state emitters such as LEDs in combination with lumiphors create white light by mixing relatively narrow wavelength bands together with spectral gaps between peaks of LEDs and/or lumiphors. The resulting light may be under-saturated with certain colors of the spectrum or oversaturated with certain colors. One way to alleviate oversaturation with respect to certain portions of the visible spectrum and thereby improve CRI includes notch filtering of LED lighting systems with an optical element (e.g., incorporating a rare earth compound such as neodymium oxide, or a color pigment) that filters light emissions so that light passing through or reflected by the optical element exhibits a spectral notch, as disclosed in U.S. Patent Application Publication No. 2013/0170199 A2 entitled “LED lighting using spectral notching” (which is hereby incorporated by reference herein). Such publication discloses that CRI and GAI values of LED light sources can be improved through use of notch filtering, such as to increase CRI from 84 to 90, and to increase GAI from 50 to 58. Alternatively, careful selection of materials used in LED lighting devices may permit attainment of CRI Ra values of 90 to 95 or more—see, e.g., U.S. Pat. No. 7,213,940, which is hereby incorporated by reference.
The art continues to seek improved solid state lighting devices providing desirable illumination characteristics, as well as improved methods for making solid state devices.